376 HYDROGRAPHICAL SURVEYING [chap. xvii. 



Now do the same for the large scale, making an equal number 

 of squares. 



It will be seen that nothing is measured, everything being 

 done by subdivision of the directing line. 



Great care is necessary to rule all these lines truly rectangular 

 and equidistant. 



Number the lines on each plan, to prevent mistakes, giving 

 the same number to similar lines. Letters may be put to one 

 .set of lines, and numbers to those at right angles. 



Then, taking proportional compasses, set them to the differ- 

 ence of the scale as ascertained by measuring the distances 

 apart of similar lines. The distance of each little detail of the 

 plan, from the nearest lines, can be put down by the same 

 distance from the similar lines on the small scale. 



Reducing is an operation demanding even more patience 

 and trouble than usual, and it is better to leave the space blank 

 than to reduce it carelessly. 



REDUCING PLANS BY PHOTOGRAPHY. 



Photography has recently been employed with great advan- 

 tage for this purpose. The method adopted by the Hydro- 

 graphical Department is to obtain a blue print of a photograph 

 of the original drawing, and to ink in such details as are con- 

 sidered necessary to be shown on the plan when reduced, 

 omitting those that are unsuitable for the smaller scale. The 

 soundings are selected with a view to avoiding overcrowding, 

 but the selection should be judicious in order to show appro- 

 priately the undulations of the bottom, so far as the reduced 

 pcale will admit ; particular care being taken that the shoaler 

 soundings are not omitted in places where it is essential to 

 indicate them. The soundings so selected should be inked in, 

 in type of such character that, when reduced to the required 

 scale, they shall be of suitable size. Names, etc., should be 

 similarly treated. A line should be drawn joining two stations, 

 as far apart as possible, on the blue print, and the distance in 

 inches of these stations from each other as they will appear 

 on the reduced plan should be furnished to the photographer. 

 The blue print inked in as described above is then brought 

 down to the required scale by photography, and an ordinary 



